Magnetic identification and separation of small parts

ABSTRACT

In the manufacture of a number of small parts, selected areas of the surface of a sheet are inspected and defective areas are magnetically identified with a specially formulated, relatively high-viscosity magnetic fluid. The sheet is the divided into a number of small parts and those parts formed from defective areas of the sheet are magnetically separated from the remaining parts. In this method, air pressure is applied to a free piston located within the barrel of a pen that forces the magnetic fluid through the capillary tip of the pen onto the selected surface areas. Methods and apparatus are provided for controlling the size and shape of the magnetic mark.

United States Patent Original application Aug. 31, 1967, Ser. No. 664,703, now Patent No. 3,572,400. Divided and this application Jan. 9, I970. Ser. No. 7,292

MAGNETIC IDENTIFICATION AND SEPARATION OF SMALL PARTS 4 Claims, 7 Drawing Figs.

U.S. Cl n 209/74\ 209/] l 18 Int. Cl a 4. B07c 5/344 Field of Search 209/8l R, 111.8,81 A, 74, 72, 73; 324/158 T. 158 D, 158 SC, l58 F [56] References Cited UNITED STATES PATENTS 3,165,8l8 1/1965 Soffaetal 324/l58D 3,503,500 3/1970 Klossika 209/74 3,507,389 4/1970 Casner et al. 209/l l 1.8

Primary Examiner-Allen N. Knowles Attorneys-H. .l Winegar. R P Miller and J. L. Landis ABSTRACT: In the manufacture of a number of small parts, selected areas of the surface of a sheet are inspected and defective areas are magnetically identified with a specially formulated, relatively high-viscosity magnetic fluid. The sheet is the divided into a number of small parts and those parts formed from defective areas of the sheet are magnetically separated from the remaining parts.

In this method. air pressure is applied to a free piston located within the barrel of a pen that forces the magnetic fluid through the capillary tip of the pen onto the selected surface areas Methods and apparatus are provided for controlling the size and shape ofthe magnetic mark LPREPARE SEMICONDUCTOR suTI [gel-i GRID LINES ON SURFACE or sucfl ILNSPECT s URFACE OF sucfl DRY MAGNETIC FLUID IiCRlBE AND BREAK SLICE INTO WAFEPH LgPARATE WAFERS MAGNETICALLYT PAIENTEDuuv 30 ml 3,623 .603

sum 1 [IF 2 LPREPARE SEMICONDUCTOR sucfl LTCH can LINES ON SURFACE OF sucfl UNSPECT 5 UR FACE OF sucfl MARK DEFECTIVE AREAS WITH MAGNETIC FLUIEI DRY MAGNETIC FLUID ISCRIBE AND BREAK SLICE. INTO wAFERs] [SEPARATE WAFERS MAGNETICALLTI INVENTORS 3.0 CASNER R. 7'. GOULSTONE R RHA N05 57 A TTORNEV MAGNETIC IDENTIFICATION AND SEPARATION OF SMALL PARTS This application is a division of application Ser. No. 664,703, filed Aug. 31, 1967 now U.S. Pat. No. 3,572,400.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the manufacture of small, lightweight, nonmagnetic parts by subdividing a larger sheet of material into many such parts. The invention is particularly concerned with an inspection system wherein parts formed from defective areas of a sheet are magnetically identified so that they can be magnetically separated from those parts that meet the inspection standards. More particularly, this invention relates to methods and apparatus for depositing a controlled quantity of a specially formulated, relatively highviscosity magnetic fluid on selected surface areas of a sheet of material, which deposit is of predetermined size and shape.

While this invention may find utility in the manufacture of many types of small parts, it is described for convenience and clarity herein with specific regard to the manufacture of semiconductor wafers, which is particularly illustrative of the practice of the invention. It is to be understood, however, that the invention is not intended to be limited to the manufacture of semiconductor wafers.

2. Description of the Prior Art It is conventional in the manufacture of semiconductive devices to subdivide a crystal of semiconductive material into a series of thin disks by making a number of parallel slices through the crystal. These disks, referred to herein as slices, are given appropriate treatment, as by doping with desired conductivity-type determining impurities, and each slice is then divided into many hundreds of small chips or wafers. The slices are usually comprised of a rather hard, brittle material, such as silicon or germanium, thus the shaping of the wafers is readily accomplished by one of several known methods, such as by sawing the slice with a diamondimpregnated wheel, or by mounting the slice on a ruling engine, scribing fine lines with a diamond stylus, and then breaking the slice along the scribed lines. In addition, ultrasonic cutting or grit-blasting techniques are used in some applications.

Prior to the final assembly of the wafers, it is necessary to in spect the wafers visually and electrically in order to identify and discard wafers that fail to meet physical and electrical specifications. Because semiconductor wafers are quite small (a typical wafer may be only 4 to 6 mils in thickness and 20 mils square), manipulation of the individual wafers during inspection and testing operations is a tedious process. For this reason, it has been found economically desirable to inspect selected areas of a slice from which semiconductor wafers will be formed, prior to the time that the slice is divided into individual wafers.

Typically, a pattern of grid lines is etched through the oxide layer on the surface of a slice to outline the specific areas of the slice from which each individual wafer will be formed. Sensing elements of a test probe are then sequentially indexed from one area to the next, and certain of the electrical properties are measured. At the same time, or during a separate operation, an operator may inspect each outlined area under a microscope for any visible flaws that may appear on the surface of the slice.

When defective areas on the surface of the slice are de-' tected, these areas are identified with a mark or symbol so that, after the slice has been divided into individual wafers, the wafers formed from these defective areas can be separated from the remaining wafers that have passed inspection.

One method of identifying the defective wafers utilizes a marking instrument, such as a pen, that, upon signal from the operator or the electrical testing device, will cause a small drop of ink to be deposited on the defective area. The ink mark provides a visual identification that enables an operator to sort out the defective wafers.

It will be appreciated that considerable dexterity is required in manual sorting of the wafers since it is possible to produce several thousand wafers from a single slice only an inch or two in diameter. In examining the wafers, the operator must view the wafers under magnification and use a vacuum pencil or similar device to pick up and manipulate each wafer. Further, since an ink dot is deposited on only one side of a defective wafer, the wafers must. be turned over and examined on both sides to ensure a thorough inspection.

It is understandable that the accuracy of this tedious and time-consuming sorting process is subject to human error and that some defective wafers may pass undetected. Therefore, in order to increase both the speed and efiiciency of the sorting operation, it has been suggested that defective areas of a slice be identified by depositing a small dot of magnetic particles on the surface of the slice. Then, after the slice has been divided into wafers, those wafers formed from defective areas can be separated from the others by magnetic means.

While this method is theoretically sound, it presents certain practical problems. First, it is difficult to deposit and confine magnetic particles to precise areas on the surface of a slice. In order to facilitate the flow of the magnetic particles through a pen tip or other applicator device, the particles must be suspended in a liquid carrier or vehicle. Since it is desirable to maximize the amount of magnetic particles contained in a given dot deposited on the surface of a slice, the solids content of the magnetic fluid should be as high as possible while still permitting flow of the fluid through an applicator tip. A high viscosity, that is consistent with flow through the applicator tip, is also needed to maintain the magnetic particles in suspension to prevent them from settling out of the liquid carrier and clogging the applicator tip. Further, if the suspension is made too thin, the fluid will flow beyond the defective area and contaminate adjacent areas. This may be a particular problem in those instances where the slice has been etched with grid lines, since these etched lines will act as capillaries to feed magnetic fluid to other portions of the slice.

Second, the magnetic suspension also must be capable of drying to a hard solid that will not smear when the wafers are touched or tumbled against each other At the same time, it is undesirable to use volatile, quick-drying vehicles in preparing the suspension, because if the fluid dries too quickly at ambient conditions, the capillaries of the marking tip will clog,

necessitating frequent shutdowns for replacement or cleaning of the marking tip.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved system for sorting a plurality of small, lightweight, nonmagnetic parts.

A further object of this invention is to provide a system for magnetically identifying selected areas of a sheet of material so that upon subdivision of the sheet into a plurality of small parts, the parts subdivided from the selected areas may be separated from the remaining parts.

A more specific object of this invention is to provide a system for magnetically identifying defective areas of semiconductor slices so that wafers formed from such defective areas can be automatically sorted from the remaining wafers.

And yet a further object of this invention is to provide an improved capillary device that will feed controlled quantities of magnetic particles and deposit them at precise locations on the surface of a sheet of material.

And still a further object of this invention is to provide a stable suspension comprised of magnetic particles in a liquid car rier that will not separate upon prolonged standing, that may be deposited with precision within a specific area, that will not flow out and contaminate adjacent areas, that will dry to a hard, nonsmearing solid, and that will feed through capillary devices without substantial clogging.

Briefly, these and other objects of this invention are achieved by means of a system that includes an improved magnetic marking fluid having a viscosity of between about 30 and 300 poises that is comprised of a suspension of magnetic particles in a substantially nonvolatile carrier liquid especially adapted to be fed by means of an improved marking device that, upon signal, will cause a metered amount of the magnetic fluid to be exprossed and deposited with precision over selected areas on the surface of a sheet of material.

In somewhat more detail, the marking device includes a free piston that, on its one face, engages the surface of the magnetic fluid contained within the marking device, and, on its other face, communicates with a source of air under pressure. The marking device is movably mounted so that it may be indexed to a position where its applicator tip is in close but nontouching proximity with the surface of the sheet of material, the amount of the magnetic fluid that is expressed, and the size, and to some extent, the shape, of the dot deposited on the surface, are controlled by regulating the delivery of the air under pressure to the disengaged face of the free piston while the applicator tip is so positioned. In a preferred embodiment of this invention, the movement of the marking device and the delivery of air under pressure are controlled by signals received from electrical testing apparatus so that a dot of magnetic fluid will automatically be deposited at any point on the surface of the material that is found to be defective by the electrical testing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flowsheet illustrating the general system of this invention;

FIG. 2 is a schematic perspective view of the test apparatus and the magnetic marking device of this invention;

FIG. 3 is an enlarged view of that portion of FIG. 2 enclosed in the phantom circle;

FIG. 4 is an enlarged schematic view, in section, of the magnetic marking device of this invention;

FIGS. 5 through 7 are greatly enlarged, somewhat schematic views, in cross section, showing the applicator tip of the marking device and the object being marked, and depicting successive stages in the marking operation.

Referring first to FIG. 1, a semiconductor slice is initially prepared by cutting it from a large crystal usually grown from a Group IV element such as silicon or germanium. After the slice is carefully cleaned and polished, it is doped with materials such as elements from Groups [II and IV of the Periodic Table to establish the desired number and types of PN-juno tions. The slice is now suitably prepared for subdivision into a number of wafers from which semiconductors, such as diodes and transistors, can be manufactured.

While the slice is being doped, a hard oxide layer may be formed on its surface. Because the oxide layer is resistant to normal scribing apparatus, such as a diamond stylus, it has been found convenient to etch a grid line pattern, as with hydrofluoric acid, on the surface of the slice. Thus, not only is the hard oxide layer removed for cutting purposes, but also the individual areas of the surface of the slice from which each wafer is to be formed will be clearly identified for inspection.

After the grid lines have been etched, the slice is mounted on testing apparatus so that is may be contacted with test probes. The slice is indexed under the probes and, at each indexed position, a separate grid area (representing a future wafer) is contacted with the test probes for measurement of certain electrical properties. At this or a later time, each distinct area is also inspected for visible flaws under a microscope. Any area failing to pass inspection is identified by marking it with a fluid suspension of magnetic particles (hereinafter referred to for convenience as a magnetic fluid").

After the inspection of the surface of the slice is complete, the slice is set aside, preferably in an oven, to allow the magnetic fluid to dry to a relatively hard, nonsmearing solid. When the magnetic fluid is dry, the slice is mounted on a ruling engine and fine lines are scribed within the etched grid lines by using a fine diamond-tipped stylus. The slice is then broken into individual wafers by any convenient technique such as by placing the slice between two thin sheets of flexible material and then running a small roller back and forth over the surface of the enclosed slice.

After the individual wafers have been formed, they are sieved to remove random small chips and pieces as well as any oversize material. The middlings are then subjected to the influence of a magnetic field and the inferior wafers containing a deposit of magnetic particles are separated from those wafers that have passed inspection.

The details of the inspection device are schematically illustrated in FIGS. 2 and 3. A semiconductor slice 10 is mounted on the upper surface 11 of a movable platform I2. It is convenient to secure the slice 10 on the upper surface II by means of a vacuum. This can be accomplished by attaching a vacuum line (not shown) to a chamber (not shown) located within the platform 12. The vacuum chamber is placed in communication with the underside of slice 10 by providing air passageways through the upper surface 11. As indicated by the arrows in FIG. 2, the platform 12 is mounted for movement in a horizontal plane, either backwards and forwards or from side to side.

As can best be seen in FIG. 3, the slice 10 is etched with grid lines l313 which define individual areas 14-14 from which individual wafers will be formed. A pair of electrodes 16 is adapted to contact, upon signal, the surface of each of the areas l4-14. These electrodes 16 are suitably connected to an electrical test set 17 that measures certain of the electrical properties of the wafer. Since the electrical testing equipment forms no basis of this invention and is well known in the art, the details of its operation and construction are not described herein.

The output from the electrical test set 17 communicates via an electrical cable 18 to a magnetic fluid dispenser control 19. An arm 21 is pivotally mounted in magnetic fluid dispenser control 19 at pivot point 20 for up and down movement and the arm 21 supports at its free end a pen 22. The pen 22 in cludes a marking tip 23 and a barrel 24 that serves as a reservoir for a supply of magnetic fluid. The upper end of the barrel 24 communicates with the magnetic fluid dispenser control 19 by means of an air line 26.

Details of the pen 22 are shown in FIG. 4. The pen 22 is comprised of a main body portion or barrel 24 having an internal bore of regular cross section, preferably circular. The upper portion of the barrel 24 is provided with screw threads 28 in order that it may be removably secured in sealing relationship with a coupling member 29. The upper portion of the coupling member 29 is drilled and tapped to receive a locking bolt 31 that enables removably securing air line 26 in sealing relationship with the coupling 29. A passageway 32 is provided through the coupling 29 to provide for communication between the air line 26 and the interior of the pen barrel 24. A fixture 33 may be provided to secure the air line 26 to the support arm 2]. A source of compressed air 34 communicates via a pressure regulator control valve 36 with air line 26 and is functionally associated with the magnetic fluid dispenser control 19 to apply compressed air at a regulated pressure to the interior of the pen 22.

The lower portion of the barrel 24 of the pen 22 is reduced in diameter and is adapted to receive the pen tip 23. The pen tip 23 preferably is a capillary feed device having an orifice 37. A suitable tip for marking wafer areas 14-14 that are approximately 20 mils square can be readily prepared by removing the plunger from a commercially available inking pen tip sold under the trade designation of a 3/0 Leroy pen. The outer diameter of this tip is approximately l0 mils and the inner diameter approximately 3.5 mils.

A portion of the interior of the pen 22 is filled with a magnetic fluid 38 and a ball 39 is placed over its upper surface. The diameter of the ball 39 closely corresponds to the interior diameter of the cylindrical barrel 24 so that it may move in fluid-sealing relationship with the innerwalls of the barrel 24. Thus, functionally, the ball 39 acts as a free piston, and when compressed air is applied via air line 26 on the upper surface of the ball 39, pressure is exerted uniformly over the entire cross sectional area of the fluid 38, tending to force the fluid from the pen tip in a manner discussed in more detail hereinafter. Other geometric shapes, such as cylinders, for example, may be substituted for the ball, provided only that the shapes conform to the interior walls of the barrel 24 and will slide in substantial fluid-sealing relationship therewith.

The free piston described above is an important element of the instant invention in that it enables the use of high-viscosity magnetic fluids, as defined more fully hereinafter. As previ ously discussed, it is desirable for the viscosity of the magnetic fluid to be sufficiently high to prevent the solids from separating from the carrier liquid, to prevent the magnetic fluid from flowing or dripping, as by gravity, from the applicator tip, and to prevent the magnetic fluid from flowing from the defective area to adjacent areas of the slice. Quite generally, a fluid having this desired viscosity will have a consistency similar to a heavy grease that will not flow to any appreciable extent due to its own weight. However, when fluids of this desired viscosity are used, it is extremely difficult to express them though the small diameter orifice 37 of the pen 22. If, for example, the pen 22 is not equipped with a free piston, such as the ball 39, when compressed air is applied from the magnetic fluid dispenser control 19 by way of the air passageway 32, the viscous fluid tends to be displaced from along the center line of the pen 22. After a number of pressure pulses have been applied, a channel is opened through the magnetic fluid 38 that extends from its upper surface to the orifice 37 at the bottom of the pen. When this condition develops, further pulses of air are ineffective to express any more magnetic fluid from the pen 22.

It has now been discovered that this difficulty can be avoided if a free piston, such as the ball 39 shown in FIG. 4, is seated over the top surface of the magnetic fluid 38 so that compressed air applied to the pen 22 will be uniformly distributed by the free piston over the entire cross section of the magnetic fluid 38, and channeling will thereby be effectively prevented. This enables the use of the desired high-viscosity marking fluids as defined below.

From the foregoing, as well as from the description that follows, it will be understood that the instant invention provides a system in which there is a cooperative relationship between the design of the free piston pen and the formulation and viscosity of the magnetic fluid.

In operation, a semiconductor slice is placed on the upper surface 11 of the platform 12 and is held in position, as by means of a vacuum. The platform 12 is initially adjusted so that the electrodes 16 are positioned over the surface of the first-inscribed area ofa row.

While the machine may be either controlled manually or programmed for automatic operation, the sequence of operation remains the same. First, the electrodes 16 move down into electrical contact with the surface of an inscribed area 14. Electrical measurements are taken and the electrodes are retracted from the surface of the inscribed area 14. The semiconductor slice is then indexed to the next position so that the immediately adjacent inscribed area in the same row is moved under the electrodes 16. Simultaneously, the area previously tested moves into a position directly under the tip 23 of the pen 22. The electrodes 16 again engage the surface of the slice 10 in order that the electrical properties of the area newly moved into position may be measured. At the same time, if the area previously tested has been found to be defective, the magnetic fluid dispenser control 19 will receive a signal from the electrical test set 17, which causes the pen 22 to be lowered and applies a dot 40 of the magnetic fluid to the defective area.

When this mark" signal is received, the support arm 21 is activated, as by a solenoid 43, to cause the pen 22 to move downward a preset distance toward the slice 10, from a normal retracted position shown in FIG. 4 to a fluid-applying position shown in FIG. 6. As the pen begins to descend, the compressed-air source 34 is activated so that compressed air at a predetermined pressure is applied to the top of the ball 39. The pressure is set in accordance with various parameters, such as the viscosity of the ink and the size of the dot 40 desired, by adjusting the regulator 36. In one typical example, using the magnetic fluid described hereinafter, the pressure is normally set within the range ofabout 10-15 p.s.i. gauge.

As the pressure is applied and the pen descends, the ink at the end of the applicator tip 23 swells by pressure into a rounded drop configuration 41 (referred to hereinafter as a bulge") depicted in FIG. 5. As this bulge 41 strikes the surface of the slice 10 (FIG. 6), the surface tension is broken and the fluid wets the surface of the slice and flows out a limited distance in all directions around the center of the tip 23. The full lowering of the applicator tip 23 assists in this regard by forcing an annular portion of the drop 41 against the surface of the slice and in spreading the fluid uniformly about the center of the tip. Preferably, the final clearance between the applicator tip 23 and the slice 10 is quite small (for example, 1-2 mils in the specific example, using the 3/10 Leroy applicator tip to make a 12-15 mil dot) to assist in applying and spreading the viscous fluid 38. It can be appreciated that the final clearance of the applicator tip 23 will control the thickness of the dot 40 since the tip 23 will cause the fluid to be extruded under the sidewalls of the tip 23 and so be flattened to a thickness approximately corresponding to the clearance.

The dimensions (diameter) of the dot 40 may be governed quite closely, such as by the shape of the orifice 37 and by other process factors such as the air pressure and the time that the pressure is applied while the applicator tip is in the down" position of FIG. 6. The longer that the air pressure is applied while the tip is down, the more fluid will flow out of the tip to increase the size of the dot 40. Preferably, the size is regulated in the specific example to be only slightly larger than the pen tip; for example 12-1 5 mils for the magnetic dot using a IO-mil. OD pen tip to mark a 20 by 20 mil wafer area. Conveniently, in marking semiconductor wafer areas with the viscous magnetic fluid of this invention the dot 40 is made as large as is reasonably practical, without causing the fluid to flow over onto nondefective wafer areas.

The down time, thus, dot size, may be controlled either manually by an operator inspecting the process through a microscope, or automatically by an adjustable timer in automatic versions of the equipment. In the specific example given, the down time is set between about 50 and 200 milliseconds and, depending on the other variables, usually about milliseconds. To the extent that the marking cycle is extremely short, the application of air pressure may be considered as a pulse," but preferably the supply of air is maintained at a constant pressure from a time somewhat prior to the tip 23 reaching its lowest point to a time just prior to the retracting of the tip. Just prior to retruction, the air pressure is shut ofi and the line 26 vented by the regulator 36, so that there will be no tendency for the fluid to continue to flow as the pen retracts. The retracting step is depicted in FIG. 7, showing the irregularly rounded dot 40 left on the slice and a rounded portion 42 constituting an approximate, slightly smaller mirror image left at the tip of the pen as the pen retracts and the fluid portions 40 and 42 are broken. The fluid 38 wets and adheres to the face of the pen tip 23 in preparation for the next marking operation. In starting up the system, several bursts of air may be required to force the fluid to wet the bottom face of the pen and to form itself into the rounded portion 42 in preparation for the subsequent formation of a bulge 41.

The above process controls are quite important in the successful practice of this invention. For example, if the final clearance between the tip 23 and the surface of the slice 14 is too great, the dot 40 will not be flattened and may flow in a slightly uncontrolled manner by wetting the surface of the slice. Conversely, if the tip 23 touches the surface of the slice, further flow of fluid 38 through the tip 23 will be blocked or inhibited and a doughnut" shaped dot will e deposited. Contact of the tip with the slice is also undesirable since it will blunt the tip and destroy its tolerances,

Also, it is important that the fluid 38 be expressed (other than the initial bulge 41) only after the final clearance between the tip 23 and the surface of the slice has been obtained. If the desired amount of fluid is expressed prior to this time, the fluid 38 will climb" the sidewalls of the tip 23. This will result in several problems such as the proper amount of fluid may not be delivered to the surface of the slice or, on the other hand, the fluid 38 may accumulate on the tip and then, after a period of time, slough ofl of the tip in uncontrolled amounts at uncontrolled intervals.

From the foregoing it can be appreciated that by adjusting such factors as the formation of the initial bulge, the final clearance between the tip and the slice, the size and shape of the tip, the time that the air pressure is applied while the tip is proximate to the surface of the slice, and the amount of the air pressure, the location and dimensions of the dot can be controlled with great precision.

The above-described procedure continues sequentially until an entire row has been tested. Thereafter, the platform 12 is indexed to the next adjacent row and the operation is repeated until all of the inscribedareas of the slice have been inspected.

The delayed action of the pen 22 in marking the slice after the inscribed area has been inspected and indexed away from the electrodes 16 is solely a matter of practicality since the space superjacent the inscribed areas is too limited to accommodate both a pair of electrodes 16 and a pen 22.

It is also desirable to inspect the semiconductor slice for visual defects. This may be done conveniently in a second operation in which the slice is mounted on a platform 12 and is indexed from position to position as described above. In this case, however, the slice is viewed under a microscope by the operator, and when a fault is discovered, a signal is sent directly to the magnetic fluid dispenser l9 and a small dot 40 of magnetic fluid is deposited on the defective area in the same manner as described above.

COMPOSITION OF THE MAGNETIC FLUID As previously mentioned, the composition of the magnetic fluid is rather critical since its characteristics are important to the successful practice of this invention. As discussed above, the magnetic fluid should have a high magnetic solids content to maximize the magnetic deposit on the wafer and it must have a sufliciently high viscosity to prevent the gravimetric separation of the magnetic particles from the carrier liquid, to prevent the fluid from flowing by gravity or dripping from the tip of the pen, and to prevent the fluid from flowing over the surface of the slice. At the same time, the viscosity of the fluid must be sufficiently low to enable the fluid to be expressed by pressure through the small orifice 37 of the capillary-marking device. it has been found that these conditions can be met if the fluid has a viscosity, as measured on a Laray Viscometer at 25 C. of between about 30 and 300 poises and more particulariy in a range offrom about 55 and 75 poises.

A further requirement for the magnetic fluid is that it must dry to a reasonable hardness so that it will not smear when the slices are handled in later processing. This requirement is complicated by the fact that conventional quick-drying, volatile fluids will evaporate at the tip of the pen and will cause the small orifice to clog quite rapidly. It has been found, however, that if comparatively nonvolatile drying or semidrying oils are used as the vehicle, the magnetic fluid will dry to a nonsmearing solid when deposited on a slice, but will not evaporate and clog the small orifice of the pen tip. I

in a suitable formulation, the vehicle may be a drying or semidrying oil, a drying or semidrying oil modified by a natural or synthetic resin, or a synthetic vehicle. Particularly suitable are the vehicles known as lithographic vehicles or varnishes comprising a base capable of fluid flow. The base may be a bodied oil, such as boiled linseed oil, or a hydrocarbon, or an oil from the synthetic resinous material such as alkyd resin or a phenol formaldehyde resin. The following are classes of varnishes that are useful in the practice of this invention: linseed-modified phenol formaldehyde varnish; bodied linseed oil varnish; maleic-alkyd varnish; pentaerythritol alkyd varnish; and hydrocarbon varnishes.

The magnetic component of the marking fluid is preferably comprised of a ferromagnetic material such as magnetic iron or iron oxide in very finely divided form. One particularly suitable iron oxide is marketed under the trade name lRN-lOO by the C. K. Williams Company, and is described as an acicular iron oxide prepared by known reduction and oxidation techniques.

In addition to the liquid vehicle and the magnetic solids, it may be useful to include a surfactant in the magnetic fluid to help the fluid wet the surface of a slice. Any of a number of well-known surfactants may be used and soybean licithin may be mentioned by way of example as being particularly suitable.

A general description of the types of magnetic fluids that may, when properly adjusted for magnetic iron and iron oxide content, be useful in the practice of this invention may be found in Shoemaker et al. US. Pat. No. 3,082,171 which is incorporated herein by reference.

The amount of magnetic particles present in the magnetic fluid is important in establishing the required properties of viscosity and flow. As discussed above, it is desired to include the maximum amount of magnetic particles that is possible without raising the viscosity of the fluid beyond acceptable limits of flow. Generally it may be said that the magnetic iron particles should be present in the liquid vehicle in an amount from about at least percent to obtain a minimum acceptable viscosity and up to about 60 percent by weight without exceeding the maximum acceptable viscosity. More preferably, the magnetic particles should be present in an amount of from about to 56 percent by weight. 5 3:] percent magnetic iron oxide by weight is almost ideal in the system for marking slices as described in detail hereinabove.

it will be appreciated that the requirements may vary from application to application, particularly with regard to the diameter of the pen tip, and that the amount of magnetic particles present in the magnetic fluid should be adjusted within the above specified limits to provide the desired degree of viscosity and flow.

The following is a specific example of a formulation for a magnetic fluid that is particularly suitable for use in the practice of this invention.

EXAMPLE Weight Percent Linseedmodified phenolic varnish 24 The above-formulated magnetic fluid has a viscosity of about 60 to poises as measured at 25 C. with a Laray viscometer. The fluid may be expressed, without dripping or running, through a 3/10 Leroy tip (3.5 mils. internal diameter) when a pressure of 10 to 12 psi. gauge is applied to the device of this invention having a free piston located over the fluid reservoir in the marking pen. When the fluid is deposited as described in detail above with the apparatus of this invention, the fluid will wet the surface of an area about l0 to 15 mils in diameter but will not flow ofi of the surface of the wafer and contaminate adjacent areas of a slice. When dried in an oven, for example, at 200 C. for 1 hour, the magnetic fluid will dry to a relatively hard, nonsmearing solid. Further, the dot will tightly adhere to the surface of the slice and will not be dislodged in subsequent handling operations, such as in braking the slice into wafers and the screening of the wafers. The marking pen may be used for periods of several days. The tip will not clog, the magnetic particles will not separate from the carrier liquid, and channels will not be opened through the magnetic fluid contained in the reservoir of the pen.

A magnetic fluid suitable for use in the practice of this invention can also be prepared by thoroughly mixing percent by weight linseed oil with a commercially available product as sold by the A. B. Dick Company under the trade designation 3-3101 Magnetic Black Ink.

From the foregoing it can be understood that the successful practice of the magnetic identification system of this invention depends upon the controlled deposit of the magnetic fluid along with the conjoint use of a free piston in the barrel of a marking pen and of the specially formulated magnetic fluid described above.

Once the defective wafers have been marked with the magnetic clots 40 and the slice has been broken into wafers, the defective wafers may conveniently be separated from the unmarked ones, without any specific orientation being necessary, by magnetic methods. Preferred magnetic sorting systems are disclosed in related, commonly assigned, copending applications of the applicants, B. G. Casner and R. T. Goulstone, Ser. Nos. 664,704, now U.S. Pat. No. 3,507,389 and 665,169, now U.S. Pat. No. 3,474,904 filed Aug. 31, 1967 and Sept. 1, 1967 respectively.

Although certain embodiments of the invention have been shown in the drawings and described in the specification, it is to be understood that the invention is not limited thereto, is capable of modification, and can be rearranged without de parting from the spirit and scope of the invention.

What is claimed is:

l. A method for magnetic sorting of a plurality of small, lightweight, nonmagnetic elements that are formed by subdividing the elements from a larger sheet of material, comprismg:

mounting the sheet on a test stand;

contacting selected surface areas of the sheet with test probes;

measuring the electrical properties of the selected areas with the test probes;

signaling a fluid dispenser control when the measurements of any areas are unsatisfactory;

introducing superatmospheric pressure air upon signal above a free position located within a reservoir containing magnetic fluid;

forcing a small quantity of magnetic fluid from a capillary opening in the reservoir by means of the air pressure exerted upon said free piston;

moving the capillary opening into close proximity with the surface of the substrate and contacting the surface with said small quantity of fluid;

continuing the introduction of the air to cause additional quantities of the magnetic fluid to be expressed from the capillary opening;

relieving the pressure on said free piston when the desired quantity of magnetic fluid has been deposited on the surface of the substrate;

moving the marking device away from the surface of said substrate;

curing the magnetic deposit on the surface of the substrate to a substantially solid, nonsmearing solid;

subdividing the substrate into a plurality of elements; and

subjecting the elements to the influence of magnetic lines of flux and magnetically separating the elements marked with the magnetic fluid from the remaining elements that have passed the electrical inspection.

2. A method according to claim 1, wherein the thickness of the magnetic deposit is controlled by the distance between the capillary opening and the substrate while the fluid is being expressed. I

3. A method according to claim 1, including the step of adjusting the magnetic solids content of the fluid to be between about 50 to 56 percent by weight and the viscosity of the fluid from about 55 to 70 poises.

4. A method according to claim 3, wherein the magnetic solids are comprised of finely divided magnetic iron oxide that is present in an amount from about 52 to 54 percent by weight.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5 5, 3 Dated November 50, y 1971 lnventor(s) Bernard G. Casner, Ray '1. Goulstone, Peter R. Hance It is certified that error appears in the above-identified parent and that said Letters Patent are hereby corrected as shown below.

F. Column 2, line 4-2, period omitted after "other" Column 3, line 8, "exprossed should be --expressed--.

line 24, "3/10" should be --/O--.

Column 6,

Column 6, line 58, "retruction" should be --retraction--. Column 7, line l, e" should be --be--.

Column 8, line 4-0, "50" should be --50 percent--.

Column 8, line #0, "55" should be -55 percent--.

Column 8, line 67, "5/10" should be 5/0".

Column 10, line 7, "position" should be --piston--.

Signed and sealed this 11 th day of July 1972.

( SEAL) Attest:

EDU JARD M.FLETCHLR, JP. BO BERT GOTTSCHALK Attesting Officer Commissioner of Patents 

1. A method for magnetic sorting of a plurality of small, lightweight, nonmagnetic elements that are formed by subdividing the elements from a larger sheet of material, comprising: mounting the sheet on a test stand; contacting selected surface areas of the sheet with test probes; measuring the electrical properties of the selected areas with the test probes; signaling a fluid dispenser control when the measurements of any areas are unsatisfactory; introducing superatmospheric pressure air upon signal above a free piston located within a reservoir containing magnetic fluid; forcing a small quantity of magnetic fluid from a capillary opening in the reservoir by means of the air pressure exerted upon said free piston; moving the capillary opening into close proximity with the surface of the substrate and contacting the surface with said small quantity of fluid; continuing the introduction of the air to cause additional quantities of the magnetic fluid to be expressed from the capillary opening; relieving the pressure on said free piston when the desired quantity of magnetic fluid has been deposited on the surface of the substrate; moving the marking device away from the surface of said substrate; curing the magnetic deposit on the surface of the substrate to a substantially solid, nonsmearing solid; subdividing the substrate into a plurality of elements; and subjecting the elements to the influence of magnetic lines of flux and magnetically separating the elements marked with the magnetic fluid from the remaining elements that have passed the electrical inspection.
 2. A method according to claim 1, wherein the thickness of the magnetic deposit is controlled by the distance between the capillary opening and the substrate while the fluid is being expressed.
 3. A method according to claim 1, including the step of adjusting the magnetic solids content of the fluid to be between about 50 to 56 percent by weight and the viscosity of the fluid from about 55 to 70 poises.
 4. A method according to claim 3, wherein the magnetic solids are comprised of finely divided magnetic iron oxide that is present in an amount from about 52 to 54 percent by weight. 